Pelletization of iron ore concentrates



Nov. 15, 1960 F. D. DE VANEY 2,960,396

PELLETIZATION OF IRON ORE CONCENTRATES Filed Dec. 23 1957 PELLETIZATIONOF IRON ORE CONCENTRATES Fred D. De Vaney, Duluth, Minn., assignor toP-M Associates, Cleveland, Ohio, a copartuership Filed Dec. 23, 1957,Ser. No. 704,609

6 Claims. (Cl. 753) This invention relates to the art of preparingfinely divided iron ore materials, e.g., concentrates, in a formsuitable for subsequent use in a blast furnace or open hearth furnace,and is particularly concerned with the pelletization (including both (a)forming into small balls or pellets and (b) indurating the pellets soformed) of such finely divided iron ore materials as comprise asubstantial proportion of a realtively non-magnetic oxidic compound ofiron including the minerals hematite, specularite (specular hematite),limonite and siderite. The invention will, in the following, bediscussed with more specific reference to pelletizing speculariteconcentrates.

The pelletization of finely divided specularite is peculiarlytroublesome because of the nature of its particles. Specularite per seoccurs in the form of well-developed platey crystals of hexagonalsystem, which crystals may be accompanied by a relatively small amountof particles finer than 1 micron (so-called colloidal fraction).Specularite is relatively dilficult to grind. In any of the systemscommonly employed for concentrating specularite which has been ground tosize of liberation-cg, flotation, or gravity separation using Humphreyspirals it is customary if not essential preliminarily to remove fromthe ground material all particles finer than 10 microns (theconventional preliminary de-sliming treatment) in order to expedite theparticular concentrating procedure which is to follow. The de-slimedconcentrate, because of its distinctly platey nature and because of theabsence of very fine particles, i practically impossible to ball up in aconventional balling apparatus (inclined rotary drum, or balling cone,or pan-type pelletizer), and any pellets so formed have exceedingly poormechanical strength both in the green (non-heat-treated) or in the fired(heat-treated) stage.

In evaluating the ballability of an iron ore concentrate, the amount ofmaterial passing a minus 325 mesh screen has conventionally been held tobe the criterion in describing the structure of a concentrate whichballs well. This is a very rough measurement. Actually, it has beenfound that the balling characteristics depend on the proportion of thematerial which is in minus 1 micron, or sub-micron, range. For example,in the case of an iron ore concentrate which is all minus 325 mesh andcontains a substantial amount of material finer than 10 microns (whichmaterial, as is, balls very Well and produces a pellet having good greenstrength), if one de-slimes this ground productremoving from itsubstantially all of its content of minus 10 micron particles thede-slimed material would not ball at all or would ball so poorly as tobe unacceptable. Conversely, a ground product containing sizes as coarseas 10 mesh and only 50% finer than minus 325 mesh, but containing about10% of material finer than 10 microns, balls readily and the pelletsusually have good green strength. The fact that no problem arises inballing concentrates from earthy hematites is explainable on the basisof substantial absence therefrom of platey crystals and of the presencetherein of a substantial content of minus 10 micron particles.

The particle size distribution of a specularite concentrate obtained,from a predominantly non-magnetic lean ore (found in the Wabush Lakearea, Labrador), by crushing and grinding to the particle size ofliberation, de-sliming and gravity concentration on Humphrey spirals maybe illustrated in column A of Table I following. For comparison, incolumns B and C are given screen analyses of two different magnetiteconcentrates.

TABLE I A B O Mesh Specular- Magnetlte Magnetlte ite Cone. Gone. Cone.

Total 100. 0 100. 0 100. 00

In connection with the foregoing Table I, it'may be noted that the datagiven in column C are illustrative of a magnetite concentratecommercially produced by Erie Mining Company, at its commercialbeneficiation plant nearvAurora, Minnesota, U.S.A., from a taconitewhose iron content consists essentiallyof fine magnetite, thebeneficiation procedure employed including the, steps of crushing,grinding and magnetically separatingthe; magnetite particles as aconcentrate. The concentration is then agglomerated by filtering, mixingthe filter cake with finely divided anthracite (about 10 lbs. per longton, dry weight, of concentrate) and bentonite (about 12 lbs. per longton, dry weight, of concentrate), balling the mixture in a balling drum,and indurating the small balls. or pellets in a shaft-type induratingfurnace at a maximum heat-treating temperature of 2350 F. or above. Inthe commercial operation aforesaid, certain practical criteria have beenset up for evaluating the mechanical strength or ruggedness of the ballsor pellets, viz.

Drop test No At least 6. Compression test, green pellets At least 8 lbs.Compression test, pellets dried at A drop' test number of 6 means that.the average green pellet can be dropped six times from a height of' 18inches before it breaks. This figure gives :an indication of theresistance of the green pellet to the breakage of pellets in passingfrom the balling drum to the first conveyor, and from one conveyor toanother, and from the last conveyor onto the furnace loader and from theloader onto the top of the charge column in the furnace: in a realsense, it is a measure of the resiliency of the green pellet.

The compression test measures the strength of the pellet, in the numberof pounds required to break the pellet. It has been found that if apellet has a compres sion strength less than 8 lbs. the loading ofmaterial on the pellet in the upper part of the furnace shaft will breakthe pellet.

In attempting similarlyto agglomerate therspecularite Drop test No. 1.5Compression test, green pellets lbs 2 Compression test, pellets dried at250 F. ..lbs 8 Compression test, pellets fired at 2350 F. lbs

The green pellets were of very inferior quality, and they could neitherbe handled by conveyors nor fired on any type of grate sintering machineor shaft-type indurating furnace to give a satisfactory result.

1 "It heretofore had been thought that the above-described disadvantagesin pelletizing specularite concentrates arose from mere particle sizeand particle size distribution, and that the way to improve theballability of the specular hematite concentrate was to subject it-or, apart of it to finer grinding whereby to increase the proportion of finersizes of particles in the material to be balled. However, it was foundthat finer grinding did not solve the problem, due undoubtedly to thefact that the more finely ground specularite, while, it is true, beingof smaller particle size, nevertheless was grainy (well crystallized)and provided little or no plasticity to the concentrate mixture to bepelletized; while the mixture containing the more finely ground materialshowed some improvement as to-ballabilitiy, the resulting pellets wereextremely fragile (would not withstand necessary handling in the greenstate) and broke down badly on firing. This unsatisfactory situationcould, to some extent, be corrected by homogeneously dispersing throughthe specularite concentrateseg concentrates a part or all of which hadbeen subjected to fine grinding, as aforesaid-a relatively very largeamount (of the order of by weight, more or less) a: bentonite. However,the inclusion in the pelleti zed product of such a large amount offoreign material of this sort was both economically and technologicallyunfeasible', because of the added cost of. the large amount of bentoniteand because the product would be adulterated with so large a proportionof non-ferrous material not desirable for burdening the blast furnace.

It has now been found that the ballability of a specularite concentratecan be greatly improved, and that the mechanical strength of pelletsconsisting largely of specularite particles can be greatly increased, byhomogeneously dispersing through the specularite concentrateprior toballinga substantial proportion of a concentrate of finely dividedmagnetite particles, either natural magnetite or the artificialmagnetite produced by subjecting an iron ore the content of oxidiccompound of iron of which is essentially non-magnetic iron oxide, e.g.,hematite, limonite, or siderite, to reductive roasting under conditionsto convert the iron oxides to Fe O The added magnetite tends to give theballs 21 structure which cannot be duplicated in a ball of speculariteonly. The magnetite particles are largely isometric (not platey),and'their inclusion in the specularite materially improves theballability of the same, and materially adds to the green-strength'ofthe pellets and to their firedstrength as well. All of the so-calledcolloidal material (i.e., material having a particle size less than 1micron diameter) naturally occurring in the raw material is retained inthe magnetite concentrate, to provide in the specularitemagnetitemixture the plasticity necessary for good balling and for good greenstrength and fired strength of the pellets.

In applying this discovery to the pelletization of a specular hematiteconcentrate, the amount of added magnetite, with respect to a unitamount of the specularite particles, may be adjusted within relativelywide limitsdepending upon the fineness of grind of the magnetiteconcentrate including particularly the relative content therein of veryfine particles minus 200 mesh plus 325 mesh, minus 325 mesh, andsub-micronparticlesfor enhancing the plasticity and promoting theballing of the composite concentrate. In this connection, it frequentlywill be found desirable deliberately to over-grind the magnetiteconcentrate portionsubstantially beyond the normal size of liberation ofthe magnetitein order to have available a higher proportion of thefinest particles.

Thus, in a situation where there is available, for the above purpose, arelatively large amount of magnetite concentrate, e.g., 75 parts ofmagnetite concentrate (the normal size of liberation of which is minus48 mesh) and 25 parts of specular hematite concentrate, the mag- Inetite will be ground to all minus 65 mesh in order to improve it as abinder. Where about as much magnetite is available as is the amount ofspecularite to be balled, it is desirable to grind the magnetite toessentially all finer than 100 mesh. Where the ratio of availablemagnetite to specularite is of the order of 35:65, the magnetite shouldbe ground much finer, i.e., to minus 200 mesh. As an extreme example,Where the ratio of available magnetite concentrate to speculariteconcentrate is only 15:85, it is necessary to grind the magnetite to allminus 325 mesh in order to provide the necessary plasticity and bondingof the particles of the composite.

As was suggested above, where a concentrate of natural magnetite is notavailable the same may be substituted by a concentrate of an artificialmagnetite produced by reductively roasting a non-magnetic iron ore(e.g., a portion of the specular hematite ore, or an equivalent amountof an earthy hematite ore, or limonite or siderite ore) prepare aconcentrate therefrom after suitably finegrinding, and admix suchartificial magnetite concentrate with the specularite concentrate.

To illustrate, the so-called Jasper ores of Michigan, in which hematiteores specularite is frequently a major constituent, may be used as anexample. Michigan Jasper ores are frequently concentrated by a flotationprocess in which, after the crude ore has been ground to liberation size(approximately minus 65 mesh), the ore is pulped, de-slimed, and thehematite recovered by flotation. It has been found virtually impossibleto ball this flotation concentrate per se, and the expensive step offurther grinding the materialto promote ballabilitydoes not completelysolve the problem, even with such further grinding and with theemployment of much bentonite in the mix the characteristics of theresulting pellets, both'as regards green strength and as regards firedstrength, are so poor as to make these measures only partiallyeffective.

However, by reductively roasting a substantial portion (e.g. from 15% to50% or more) of the Jasper ore or its concentrate, over-grinding theresulting artificial magnetite, recovering a magnetite concentratetherefrom and adding the latter to the Jasper concentrate, the resultingcompositev concentrate has been found to have good ballingcharacterstics and to yield pellets having good green strength (and, aswell, good fired strength).

As will be: appreciated from the foregoing, in lieu of reductivelyroasting the Jasper ore, whereby to obtain a.

.material from which a magnetite binder may be recovered, one mayreductively roast an earthy hematite and from the roasted materialmagnetically derive a magnetite concentrate suitable for admixture withthe Jasper nonmagnetic concentrate. Or, where an ore material containingnatural magnetite is available, the same may be finely-ground andmagnetically concentrated to yield the necessary binder for the Jasperconcentrate.

Where reductive roasting is'restorted to, the same may be carried outaccording to known technique. For example, the procedures disclosed inUS. Patents Nos. 2,528,552, 2,528,553 and 2,670,946 may be followed forthis purpose. As an alternative, the non-magnetic iron ore from whichthe artificial magnetite is to be prepared may be worked up by crushingthe ore to suitable fine size and subjecting the crushed material tofluo- 5 solids reductive roasting, followed by further fine grinding, ifnecessary, and magnetic concentration.

It should be noted that, whatever the mode of reductive roastingselected, the roasted product is much easier to fine-grind than is thenon-roasted material.

The magnetite component of the mixture of non-magnetic and magnetic ironore materials may, as stated previously, be formed from a portion of thenon-magnetic iron ore material by reductively roasting said portion,grinding the reduced material to very fine particle size, andmagnetically separating the magnetite. In carrying out this procedure inthe case of a boggy, wet or highmoisture, low-grade, ore material it maybe found unfeasible or impossible to roast the material in its naturalstate, or to screen the raw material and thereafter roast the same byconventional methods. In such event, a portion of the high-moisture orematerial can be balled up, in any known type of balling device such as aballing drum or balling cone, whereby to form the material into smallpellets, and the balled material subjected'to a reductive roast underconditions to convert the content'of non-magnetic oxidic compound ofiron to Fe O In case these pellets are too fragile to be reductivelyroasted in a shaft-type furnace, they may be reductively roasted in ashallow bed (so as not to impose crushing loads on the feeble pellets)as on a horizontal traveling gratetype of furnace.

The above described concept of improving the ballability of iron oreconcentrates consisting essentially, or largely, of speculariteparticles can be applied to other specularite concentrates than thoseobtained by flotation concentration; thus, they can be applied withequal elfectiveness to a specularite concentrate obtained by a fineparticle gravity-type concentration procedure such as the use ofHumphreys spirals.

Example 1 As an example of the balling phase of the invention, 70 partsof a specular hematite concentrate, having the size analysis given,under specular hematite conc., in Table I hereof, were admixed with 30parts of a magnetite concentrate, having the size analysis given incolumn C of Table I. The specularite concentrate had a moisture contentof 6.0%, while the magnetite concentrate had a moisture content of about9.5%. To the mixture there were added, in pounds per long ton ofmixture, dry weight, bentonite -20 and soda ash (dispersing agent)1.0-1.5, together with an amount of finely divided solid carbonaceousfuel (coal, or coke) appropriate for the ensuing induration process. Inthe case of shaft furnace induration about 20 lbs./l.t. of finelydivided fuel were used, whereas in the case of induration on a travelinggrate type of apparatus 40-60 lbs./l.t. of fuel were called for. Theamount of soda ash employed was dependent upon the pH of the mill waterused.

The above components were homogeneously admixed, and the mixture wasballed up in an inclined rotary balling drum, under conditions to workthe mixture well for development of optimum plasticity. The mixtureballed well, and the resulting pellets had ample green strength fortransportation to the ensuing induration operation without damage.

Comparative strength data for these pellets averaged:

Drop test No. 6.1 Compression test, green pounds 10.2 Compression test,dried at 250 F do 36.0

Compression test, fired at 2450 F. do 1250.0

Example 2 The starting material was a lean iron ore from the 6 Wabushdeposit, central Labrador, analyzing 37.3% Fe, the iron content of theore occurring essentially in the forms of (a) specularite and (b)magnetite, in the approximate ratio of 7 parts specularite to 3 parts ofmagnetite, with a relatively small content of limonite.

The ore was crushed, and dry ground to approximately all minus 35 meshin an Areofall mill, pulped with water, and concentrated magnetically toobtain two intermediate products, viz. (a) a non-magnetic portioncontaining essentially all of the specularite, and (b) a magneticportion containing essentially all of the magnetite.

The non-magnetic portion represented 82.6% by weight of the total, andassayed 32.61% Fe and 48.99% Si, together with a small percentage of anon-magnetic tailing from'a concurrent concentration of a magneticportion of the total ore. This non-magnetic portion was des-limed andthen concentrated by banks of Humphreys spirals in series. The resultingspirals concentrate represented 3l.84% by weight of the total crude andhad an analysis of 63.64% Fe and 4.89% Si. The sizing analysis of thisproduct is given (under specularite cone) in Table I above.

Balling tests on the above non-magnetic concentrate showed that it wascommercially impossible to ball it per se in any known type of ballingequipment (inclined rotary balling drum, balling cone, or pan-typepelletizer); also, that what balls were rolled up were too fragile to beindurated in any known type of induration equipment (shaft furnace,up-draft or down-draft horizontal grate furnace, or combinationgrate-and-kiln).

The minus 35 mesh magnetic portion contained 62.03% Fe and 10.02% Si andrepresented about 31% by weight of the recovered iron (the speculariterepresenting the remaining 69%): it was of a grade sufficiently high tobe marketable. However, in order to obtain sufficient fines to permitballing a mixture of this concentrate and the-above describedspecularite concentrate, the magnetite concentrate was further ground,in a ball mill, to minus mesh and re-concentrated magnetically. Thesizeanalysis of this final magnetite concentrate is given in column B ofTable I above. It is to be noted that this magnetic concentrate had beenground to a size materially finer than that necessary for liberation(i.e., finer than that necessary to produce a good grade of marketableconcentrate) or for pelletizing it (if pelletized alone), thisover-grinding being done purely to provide suificient finesincludingespecially particles less than 10 microns in diameterto promote thenecessary binding functions with respect to the concurrently producedgranular hematite concentrate described above.

The relative dry weights of the solids of the two concentrates were:

31.23% magnetite conc., and 68.77% non-magnetic spirals conc.

After filtering, the two concentrates were admixed, and to the mixturethere were added 12 lbs/long ton of bentonite and 1.5 lbs./l0ng ton ofsoda ash (as disperser for the bentonite), together with 25 lbs./l.t. offinely divided anthracite coal. These components were very thoroughlymixed together (by the aid of a mixer-muller) until the mixture washomogeneous: it had an average moisture content of 8.5%.

The mixture was then balled in an inclined rotary balling drum, 3 ft.diameter by 8 ft. long, such as that described and claimed in my US.patent application Serial No. 211,290, filed February 16, 1951, nowPatent No. 2,831,210, granted April 22, 1958. The mixture balled verywell, forming pellets varying in diameter between 1.5 inches andone-fourth inch. These pellets, because of their content of finelyground magnetite in association with the small amount of bentonite, wereof ample green strength to undergo transportation. from drum toindurating furnace without breakage or distor tion. The pellets were fedonto the top of a column of similar pelletsin a generally cylindricalvertical shafttype indurating furnace, '30 inches diameter, embodyingthe features of the furnace described in my U.S. patent applicationSerial No. 590,358, filed June 11, 1 956, 'now Patent No. 2,816,016,granted December 10, 1957, at the approximate rate of 1800 lbs., dryweight, per hour. The temperature of the heating gas flowing from thecombustion chambers was held at approximately 2400" F and atmosphericair was introduced at the 'following rates:

C.f.m. At the bottom of the shaft 225 Into combustion chambers 105 Thezone of maximum temperature was approximately 2450 F., and thetemperature was substantially uniform (in the pellets column) from aho'r'iz'on about one foot down from the "stockli'ne to the inlets fromthe combustion chambers, a vertical distance of about 6 feet: below theinlets, the temperature of the pellets column sloped downwardly throughthe cooling section of the furnace to an exit temperature ofapproximately 350 F. The spent gases exited from the s'tockline at about180 F.

For maintaining the above combustion chambers temperature about 2.5gals. of fuel oil, per long ton of finished product, were burned in thechambers, this amount of fuel oil together with the 25 lbs./l.t. of coalgiving a total B.t.u. consumption of 612,500 per long ton of finishedproduct. The oxidation of the magnetite content of the pelletscontributed approximately 105,000 -B.t.u.s per long ton of finishedproduct.

Average comparative strength data for these pellets did not differsignificantly from those given in Example 1 above.

The finished product, when subjected to the A.S.T standard coke tumbletest, gave a hardness number of 84.0 using a 10 meshscree'n as the indexfigure.

The data of the above example sharply distinguish from those obtainingin the case of the pelletization of an all-hematite concentrate relyingwholly on imported heat. In the latter situation, it is necessaryinorder to indurate to a hardness acceptable for shipment of the productto the lower lake ports-to push the maximum temperature to 26002700 F.in order to obtain the slag bond which is all that one can rely on insuch an operation. This difference is critical, because the temperaturelevel 2600-2700 F. is at the edge of the range where substantial actualmelting occurs, and hence-satisfactory commercial operation is verydifficult if not impossible to accomplish. By resorting to the novelmeasures set forth in the above specific example, in which the maximumtemperature was 2450 R, one can operate at a temperature safely belowthe-fusing temperature of the hematite-silica concentrate, and henceenjoy uninterrupted operation of the indurati'ng process with productionof fired pellets which are essentially discrete.

It has been found, further, that the induration of pellets comprisingnon-magnetic oxidic compounds of iron- -eg. concentrates of earthyhematite, concentrates of oxidized (non-magnetic) taconite, as well asthe abovedescribed concentrates of Jasper ores and similar oresconsisting largely of specular hematite-can be very materially improved,and that fired pellets of better characteristics can be produced, by theinclusion in such pellets, in homogeneous admixture with thenon-magnetic particles, of a substantial proportion of natural orartificial magnetite along un'th whatever other additives may beincluded in the mixture. The expression other additives is meant toinclude, but not to be limited to, a conventional binding agent, eig.,starch, bentonite, waste sulfite liquor product or the like, and alsofinely divided solid carbonaceous fuel.

The homogeneous incorporation of finely divided ma'gnetite in themixture to be pelletized not only verym'aterially improves theballability of the non-magnetic (cg. specular hematite or earthyhematite) particles, as above described, and materially increases thegreen strength of the resulting pellets, but also promotes betterihduration of the particles and a substantial improvement intheir firedstrength.

The intimation of iron ore pellets whose content of o'xi'dic compound ofiron consists almost wholly of a non-magnetic oxide of iron has long.been known to be much more difiicu lt than indurating pellets ofmagnetite. It heretofore had been proposed to effect the induration ofsuch non-magnetic ore pellets by including within them a relatively verylarge amount of solid carbonaceous fuel wherebythrough the'r'nedium ofthe heat of combustion of the fuel throughout the structure of thepellet-to bring all parts of the pellet to the necessary hardeningtemperature.

I have found that substitution or finely divided magnetite for a part ofthe'solid carbonaceous fuel content of these pellets produces a new anddifferent result, arising from two phenomena. In the first place, thetemperature level at which oxidation of magnetite to hematite is activeis materially higher than is the ignition temperature of solidcarbonaceous fuel, which circumstance brings it about that the heat ofthe magnetite oxidation is. developed in the pellets column or bed at alevel somewhat farther down from the top of the column or bed than thatat which the fuel begins to burn. Added coal (or, coke) dust, because ofits relatively low ignition,

tends to burn-in the pellets in the upper few inches of the bed.Although the heat so liberated is beneficial (in driving off moistureand in pre-heating the pellets), it does not contribute desirably to themaximum temperature necessary or desirable for thorough induration ofthe pellets. The finely divided magnetite, on the other hand, begins tooxidize with fair rapidity at about 800 F. with desirably rapid releaseof heat; that is to say, its heat is provided at a level (in the bed)where one wants to develop the peak temperature. This circumstance isadvantageous in most cases, because it normally results in less loss ofheat to the exiting gases and it tends to promote a more thoroughheating of the pellets. In the second place, the inclusion of finelydivided magnetite throughout the structure of the pellet brings it aboutthat during the ensuing induration the magnetite undergoes grain growth,so that there is developed within the pellet a network of binder whichmaterially enhances the ruggedness and hardness of the fired pellets.Addition of finely subdivided magnetite to the hematite not only contributes heat of oxidation'where it is most needed but, moreimportantly, it provides about the coarser hematite particles a layer orfilm of magntite which, by reason of the high-temperature oxidation andresulting grain growth, enmeshes the hematite particles in a crystallinecomplex formed in situ about the hematite particles resulting in a verystrong bonding of all particles together within the pellet. For both ofthese reasons, magnetite within the pellets functions in a different waythan coal and brings about a result distinct from that realizablethrough the use of solid carbonaceous fuel only, no matter how large aproportion of the latter is relied upon. In this connection, it shouldbe noted that the grain growth function of the magnetite is particularlyimportant.

Further improvement in the pelletization is realized by using themagnetite in very finely divided form-generally, more finely dividedthan would be necessary for liberationwhereby a unit amount by weight ofthe magnetite may more extensively and intimately be associated with thenon-magnetic iron ore particles of the pellet. This improvement is notduplicated by omitting the magnitite and merely more finely grinding thenonmagnetic iron ore particles to be pelletized.

eeeasee' While the above advantages are particularly important when theinduration is being carried out in a vertical shaft-type indnratingfurnace, they apply also in grate furnace indurating furnace procedurebecause here, also, the heat line moves down (or, up) through a bed ofthe pellets. The heat of oxidation of magnetiteassociated with thenon-magnetic iron ore particlescontributes to the induration of thepellets in horizontal traveling grate furnaces of the downdraft andupdraft types and also in kiln induration.

Moreover, to the extent that magnetite rather than coal contributes thewithin-the-pellets heat, the surrounding gases are maintained morehighly oxidizing.

Fired pellets of the hematite-magnetite mixture result in less breakageand create less flue dust and the pellets are more dense than are firedpellets of hematite in the induration of which solid carbonaceous fuelwas relied upon as the sole source of within-the-pellets heat, and thereis less dilution by reason of foreign matter (i.e. ashes).

The operating technique for indurating pellets of hematite-magnetitemixture is generally the same as that practiced in induratingall-magnetite pellets. The use of some coal (or other finely dividedsolid carbonaceous fuel) within the pellets is unavoidable in order tosupplement the heat derived from oxidation of the magnetite content ofthe composite. The temperature of the oxidizing heating gas (e.g., ofthe highly heated air) used is to be regulated with respect to the heatto be derived from the magnetite plus the coal so that the pellets arethoroughly heated, throughout, at a temperature close to but safelyshort of that temperature at which the pellets would fuze together.

While the induration of pellets composed of hematite and finely dividedmagnetite is, herein, described with particular reference to use of ashaft furnace, it is to be understood that the inclusion of magnetite isof advantage when indurating in an updraft or downdraft horizontaltraveling grate furnace or in a rotary kiln-type of furnace: the samephenomena which give strength as regards shaft furnace operationgenerally apply. This concept would, also, be helpful in indurating by acombination of horizontal grate and kiln.

As was intimated above, the amount of solid carbonaceous fuel'used is tobe varied to complement the particular amount of magnetite present inthe pellets, and also with regard to the type of indurating procedure tobe followed. Thus, if the mixture were 85% hematite and only 15% finelydivided magnetite, the coal content would have to be of the order of 40lbs./l.t. of ore concentrate: where the mixture was 5050, about 12l5lbs. of coal would be used, while for a 75 magnetite-25 hematitecomposite the coal content could be reduced to about lbs./l.t.

The finely divided magnetite can be either natural or artificial.

Illustrative of What can be accomplished with a different type of ore, amethod for treating a Michigan Jasper ore Will now be given.

Example 3 The ore consisted (besides gangue material) mainly ofspecularite, and had an Fe content of about 36%. It was crushed to about1 inch, and screened at three-eighths inch.

The amount of plus three-eighths material was approxi mately 40% byweight of the total. This coarse fraction was reductively roasted in ashaft furnace, converting substantially all of the specular hematite tomagnetite. The roasted ore was then ground to minus 150 mesh, and wasconcentrated magnetically, giving a concentrate assaying 67% Fe and 4.8%Si and representing a weight recovery of 51%.

The minus three-eighths fraction (amounting to 60% by weight of thetotal) was ground to minus 65 mesh,

and was concentrated by anionic flotation. The flotation concentnateanalyzed 63.3% Fe and 8.8% Si, and represented a weight recovery of 49%.

The two concentrates were filtered, and the filter cakes (moisturecontent about 8.0%) were thoroughly mixed, and to the mixture there wereadded 12 lbs./l.t. of bentonite and 20 lbs./ l.t. of finely dividedanthracite. These components were thoroughly mixed together, with theaid of a mixer-muller, and the homogeneous mixture was balled in 'aballing cone. The pellets, which were all minus 1 inch and plusone-fourth inch in diameter, were fed to the stockline of the samefurnace as that described in Example 1, and at the same rate, and underthe same conditions of temperature and rate of blowing.

The content of finely subdivided artificial magnetite made it possibleto form green balls of satisfactory properties, and a fired ballsatisfactory for transport and for steel plant use.

The fired balls when subjected to the A.S.T.M. standard coke tumble testgave a hardness of 85.5 using a 10 mesh screen as the index figure.

In a companion run, in which the only condition changed was that themagnetite concentrate was so finely ground that of it consisted ofparticles finer than 325 mesh, the comparative strength data of thepellets averaged as follows:

Drop test No. 7.5 Compression test, green pellets ....lbs 13.4Compression test, pellets dried at 250 F. lbs 42.0

Compression test, pellets fired at 2450 F. lbs 1,280

These data illustrate the fact that a product of improved quality can bemade by insuring that a large part of the magnetite content is veryfinely ground, much finer than is needed for concentration purposes. Inother words, smaller percentages of extremely fine grained magnetite canbe made to serve essentially the same purposes as a larger percentage ofa relatively coarser magnetite.

It should be mentioned, in connection with the above specific examples,that the over-grinding of the artificial magnetite was for the purposeof providing suflicient ultra fines to give a network of graingrowth-promoting crystals (of hematite derived from oxidizing themagnetite) for bonding the hematite particles. The same result wouldhave been realized had finely divided natural magnetite of the sameparticle size distribution been used in lieu of the artificialmagnetite.

In repetitions of the process of the above specific examples it has beenestablished that the magnetite addition is useful not only in the caseof hematite (earthy or specular) but also in the cases of hydrous ironoxide such as limonite, boggy ores and ores containing iron carbonate(siderite).

Hematite concentrates other than specularite concens trates alsofrequently are diflicult to ball and pelletize. This difficulty may comeabout not only because the mineral consists of hematite but also becauseof the coarse ness of the concentrate and of a lack of fines due to theparticular concentration process utilized. An example of hematitematerial difiicult to ball and pelletize by the usual procedures is atype of concentrate produced in the recovery of iron oxides fromtailings currently produced or from tailings basins that haveaccumulated through the treatment of Mesabi Range iron ore.

In the recovery of iron values from a tailings basin in the WesternMesabi District it was found that an acceptable recovery and grade ofconcentrate could be obtained by treating the tailings material by acombined process in which Humphrey spirals were used to recover thecoarser portion of the tailings and a flotation process was used torecover the fine mineral values. It was necessary to deslime the fineore prior to flotation in order to make an effective separation. Thecomposite concentrate of the spiral and flotation concentrate is shownin the middle column of Table II following:

mana e TABLE II Percent Weight Combined Mesh Spiral'and MagneticFlotation Taconite Hematite Concen- Concentrate trate where it may benoted that the product had material in it as coarse as mesh but had only7.5% finer than 325 mesh. Because of the absence of fines it wasimpossible to ball or agglomerate this product.

However, it was found that if some finer ground magnetite concentratewere added to this product the resulting mixture could be both balledand agglomerated in a commercially acceptable manner. Tests were made inwhich a mixture of the spiral and flotation concentrate, assaying 58.5%iron and 7.5% silica, to the amount of 60% was mixed with 40% by weightof a magnetite concentrate made from taconite and ground to a size finerthan'usual, that this mixture could be satisfactorily balled a 'ndagglomerated. The analysis of the magnetite concentrate was 60.5% ironand 7.2% silica. Its sizing analysis is shown in Table II. Thepredominate minerals in the composite of spiral and flotationconcentrates were earthy hematite and limonite. This 60-40 mixture wasthoroughly mixed and filtered to give 'a filter cake containing 7.5%moisture, and to the filtered mixture were added 12 pounds of bentoniteand 26 pounds of coal per long ton, dry weight, of the concentrate. Theadded magnetite gave sufficient plasticity to the admixture so that itwas possible satisfactorily to ball this material in a standard ballingdrum. The resulting balls were then fired to a temperature of 24-50Fahrenheit in a shaft furnace of the type previously described andsatisfactory pellets were made.

By the use of this technique it was possible to produce a satisfactoryproduct from the spiral and flotation concentrate without regrinding itfor the production of fines. The fired pellets product was also superiorto fired pellets made from ground hematite alone, because of thenetwork, around the hematite particles, of hematite which had beenformed from the magnetite contentduring the heat-treatment, whichnetwork gave additional strength to the pelletized and induratedproduct.

On the Mesabi Range there are available tonnages of ore which have adesirable dry analysis but because of their high moisture content thenatural iron content is low and they are therefore not economical tomine. Associated with such ores is material of a like nature but whichhas undesirable amounts of silica which must be removed to make the oresalable. Typical of such ores are those found in the Carmi Mine nearHibbing, Minnesota. Examples of typical ores from this property, one ofwhich requires concentration and one needs only to be dried andagglomerated, are given below:

Ore Analyses Per- Per- Per- Per- Percent cent cent cent cent Nat. IronSilica Alurn- Ign. Moist. Iron ina Loss Qt fl "N".'- 46. 45 16; 36 3. 734. 14:2 39. 85 ore '12, 57.65 7. e1 4. as 4.22 17.8 47. as

These orescan be treated as followsz Ore A which contains undesirableamounts of silica and which is quitesticky -in nature is crushed toapproximately /2", either by rolls or by hammerrnill, and is then'm'ixedwith about 3% anthracite fines and then rolled, in a balling device ofany conventional type, into small balls or pellets. Because of thesticky nature of the ore, the material balls readily even though some ofthe particles are as large as /2". These balls are then given a magneticroast on a special type of horizontal traveling grate which is totallyenclosed and through which reducing gases-'-either carbon monoxide orhydrogen or a mixture of the two-are caused to pass through the sinterbed. The temperature of the ore on the grate is held well below-fusiontemperatures, and under properly controlled conditions the primary ironminerals in the ore of hematite and limonite are converted almostentirely to the mineral magnetite. The balled product retains itsoriginal structure in the passage through the machine, and is cooledupon discharge in a reduced atmosphere below the temperature ofre-oxidation. This roasted product is then wet ground to approximatelymesh, and concentrated by conventional types of low intensity magneticseparators. By so doing it is possible to obtain a magnetite concentratethat has a content of 59.15% iron, 7.52% silica and 3.2% alumina, and1.05% ign. gain. I then take ore B, which has an acceptable chemicalanalysis except for its moisture, and crush it to about /2 as was donewith-ore A. To this crushed product I add about 35% by weight of thefine ground magnetite concentrate made from ore A. To this mixture ofthese two concentrates I add 12 pounds of bentonite and 1 /2 pounds ofsoda ash per long ton, dry Weight, of the mixed ore and concentrate, andafter admixing all ingredients I ball this material in a ballingmechanism. Because of the sticky nature and high alumina content of oreB together with the fine concentrates from ore A this mixture ballsexceptionally well. After balling the material is fired either in ashaft-type indurating furnace or on a traveling grate. The presence ofthe fine magnetite in the balls supplies heat to the indurating reactionand serves to act as a bond, holding together the particles of hematite.

Moreover, the fine magnetite content of the ball isduring the indurationprocedure conducted in the presence of a hot gas mixture having adefinitely oxidizing efiect-' converted to hematite and undergoes adefinite grain growth, with the result that there is produced throughoutthe ball a network of bonding material between the (original) hematiteparticles bridging and cementing to neighboring grains through themedium of the ferric oxide produced by the oxidation of the finemagnetite which had been added prior to balling. This desirable resultis illustrated in the accompanying drawing, in which Figures 1 and 2 arephotolithographic reproductions or photomicrographs (taken at amagnification of 340) of typical cross-sections of indurated balls (orpellets) produced in the experiment disclosed in specific Example 1above. Figure 1 shows the coarse particles of specular hematite bondedtogether by finer hematite resulting from the oxidation of the magnetitecontent of the original mixture from which the ball had been formed. Thebright white portions are relatively large particles of specularhematite, a typical particle being marked with the letter H. The darkgrey material, marked with the letter L, is the Lucite mounting materialused in preparing the cut-open pellet for polishing. A small particle ofquartz is indicated at Q, while the reference M leads to a relativelylarge particle of magnetite which had not-during the indurationtreatmentheen completely converted to Fe O Figure 2, similarly marked,strikingly shows how the large specularite pieces are held together by anetwork bond of cemented-together finer particles of Fe O derived fromthe fine magnetite content of the original mix.

Both views indicate that the bond between the ore particles occurs bybridging and cementation to neighboring grains through the medium offerric oxide produced by the oxidation of the original content of addedfinely divided magnetite, which bridging, it is to be remembered, occursin three dimensions. It is to be noted that there is substantially noindication, in either view, of any slag bonding: the bonding is throughan intergrowth of hematite particles produced in situ amongst the largerspecularite particles.

While the showing made in Figs. 1 and 2 pertains directly to Example 1,it equally applies to the other examples and is typical of the resultsflowing from the application of the principles of the present inventionto the pelletization and induration of hematitic ore materials.

I claim:

1. In the agglomeration of a concentrate of a platey, granularnon-magnetic iron ore containing less than by weight of particles finerthan 10 microns involving the step of forming a mixture of theconcentrate and finely divided solid fuel into small balls by a rollingoperation, the improvement which consists in homogeneously incorporatinginto said mixture finely divided magnetite in an amount by weight offrom to 75% of the total iron oxide of the resulting mixture, theparticles of said finely divided magnetite being substantially all finerthan 65 mesh and including from about 85% to about 94% of minus 200particles.

'2. In the agglomeration of a concentrate of granular hematitecontaining not more than a few percent of par ticles finer than 10microns involving the step of forming a mixture of the hematiteconcentrate and finely divided solid carbonaceous fuel into small ballsby a rolling operation, preliminary to a heat-treatment, the improvementwhich consists in homogeneously incorporating into said mixture amagnetite concentrate composed of particles most of which are of a sizesubstantially smaller than are the hematite particles and which includea substantial proportion of particles finer than 10 microns, the amountof magnetite concentrate being from about 15% to about 75 by weight ofthe total iron oxide content of the resulting mixture.

3. The improved process defined in claim 2, characterized in that themagnetite concentrate is derived by reductively roasting a non-magneticiron ore, finely grinding the roasted ore to an extent more thansufiicient to liberate the magnetite particles, and concentrating thefinely ground ore.

4. Process of working up a concentrate of specular hematite ore into aform suitable for agglomeration, which consists essentially inhomogeneously admixing with the specular hematite concentrate from about20 to about pounds, per long ton, of solid carbonaceous fuel and from15% to by weight of a magnetite concentrate composed of finely groundparticles most of which are of a size materially smaller than are thespecular hematite particles and which include a substantial proportionof particles finer than 10 microns, and balling up the homogeneousmixture by a rolling operation into pellets.

5. Process as defined in claim 4 in which the fineness of grind of themagnetite is in inverse ratio to the proportion of magnetite to specularhematite in the mixture.

6. Process of beneficiating a lean iron ore the iron content of which ismostly in the form of specularite, which comprises crushing and grindingthe ore to the extent necessary for liberation of the specularite,separating from the ground ore a concentrate of specularite, adding tothe specularite concentrate from about 20 to about 60 pounds, per longton, of finely divided carbonaceous fuel, bentonite, and from 15% to 75%by weight of a magnetite concentrate composed of particles of a sizesubstantially smaller than are the specularite particles and whichinclude a substantial proportion of particles smaller than 10 microns,homogeneously admixing, balling up the homogeneous mixture into smallballs by a rolling operation, and subjecting the small balls toinduration treatment under conditions to effect oxidation and graingrowth of the magnetite particles amongst the specularite particles.

References Cited in the file of this patent Mining Engineering 1), May1950, pages 56l563. Ban et al.: Mining Engineering (2), August 1953,pages 803-811.

1. IN THE AGGLOMERATION OF A CONCENTRATE OF A PLATEY, GRANULARNON-MAGNETIC IRON ORE CONTAINING LESS THAN 10% BY WEIGHT OF PARTICLESFINER THAN 10 MICRONS INVOLVING THE STEP OF FORMING MIXTURE OF THECONCENTRATE AND FINELY DIVIDED SOLID FUEL INTO SMALL BALLS BY A ROLLINGOPERATION, THE IMPROVEMENT WHICH CONSISTS IN HOMOGENEOUSLY INCORPORATINGINTO SAID MIXTURE FINELY DIVIDED MAGNETITE IN AN AMOUNT BY WEIGHT OFFROM 15% TO 75% OF THE TOTAL IRON OXIDE OF THE RESULTING MIXTURE, THEPARTICLES OF SAID FINELY DIVIDED MAGNETITE BEING SUBSTANTIALLY ALL FINERTHAN 65 MESH AND INCLUDING FROM ABOUT 85% TO ABOUT 94% OF MINUS 200PARTICLES.